1. Field of the Invention
The present invention relates to printing systems and, more particularly, to an apparatus for transferring a thin layer of material onto indicia that are printed, copied or otherwise affixed onto the surfaces of sheet-like articles.
2. Description of the Prior Art
The process of applying coatings to printed indicia for the purpose of coloring, decorating, texturizing or otherwise highlighting the indicia is known in the field of printing. One general class of methods commonly used for highlighting such indicia employs a special transfer sheet that includes a transferrable layer of the decorative material and a means, called a transfer unit, to selectively transfer the material onto the indicia. The transfer sheet is usually brought into contact with the indicia while the transfer unit applies heat and pressure, causing the transferrable layer to adhere to the indicia as the layer is released from the transfer sheet. The transferrable layer may be a decorative metallic foil, such as gold or silver as described in U.S. Pat. No. 4,868,049, or it may be a plastic foil, such as mylar or the like. Detailed descriptions of such prior art methods and devices may be found in the following U.S. Pat. Nos. 4,724,026; 3,519,512; 4,866,539; and 4,760,467.
The '026 patent is directed to a process for transferring metallic foil onto a xerographic image. The '512 patent discloses a machine that applies heat and pressure to a transfer sheet for applying designed indicia to articles of various shapes. The '539 and '467 patents disclose computerized foil systems that include a transfer unit mounted at the output of a paper copier or printer. The '049 patent discloses a transfer foil sheet which may be used in the process described in the '026 patent. The transfer units in the '539 and '467 patents include a plurality of rollers that automatically superimpose foil onto indicia-carrying paper. Heat and pressure are applied to the foil by means of a heated solenoid-activated roller to cause the foil to transfer onto the indicia. The foil and material onto which it is transferred are transported through the transfer unit by the interaction of a system of gears.
Although such prior-art printing systems and methods have served the purpose, they have not proved entirely satisfactory under all conditions of service for the reasons that considerable difficulty is often experienced in obtaining a high-quality product in printing systems having a high throughput and prior-art systems have been particularly prone to paper and foil jams and creases. While there is a clear need for improvements in transfer unit technology that improve printing and copying quality, no satisfactory system for doing so has yet been devised. Ideally, such a system would be self contained; would produce a consistent, high-quality product; would have the capability of transferring foil or other transferrable materials at rates equal to or greater than the throughput rates of the printer or copier which it serves; would not cause the paper or foil to jam or crease; would operate dependably with consistent quality and throughput when processing a variety of printed substrates and transfer sheets; would not overheat; would produce little or no electrical interference, noise, or other environmental problems; and would be simple in construction and inexpensive to operate.